The present invention relates generally to control systems for industrial automation applications, and, more particularly, to safety isolation systems adapted to connect, disconnect, and isolate direct current (DC) electrical power.
In a variety of environments, including for example industrial automation environments, there is a need for control systems that are capable of governing the operation of one or more pieces of industrial equipment or machinery in a manner that is highly reliable. Such control systems often employ a high degree of redundancy in their various circuits and other components, so as to guarantee or nearly guarantee that the control systems will achieve intended goals in operating the controlled equipment/machinery and, in the event of failures, that the control systems will operate in such manners that the control systems and the controlled equipment/machinery enter predicted failure states.
Among these control systems are systems generally referred to as safety isolation systems that are designed to disconnect, ground and otherwise isolate controlled equipment/machinery from one or more power sources in a predictable, reliable manner. Such control systems reduce the chance that the controlled equipment/machinery might be unintentionally restarted at times when it is being accessed by repair personnel or technicians for purposes of repair or modification, and thereby enhance the confidence and rapidity with which such personnel can accomplish such repairs/modifications. The power sources from which the controlled equipment/machinery are isolated by these control systems can include any of a number of power sources including, for example, electrical, pneumatic and hydraulic power sources.
Referring to FIG. 1, one prior known control system of this type is the ElectroGuard™. Bulletin 2030 Safety Isolation System available from Rockwell Automation, Inc. of Milwaukee, Wis., the beneficial assignee of the present application. This control system, shown in FIG. 1 as safety isolation system 10, includes both an electric power isolation system 12 and a pneumatic (or hydraulic) power isolation system 14, and operates as follows.
When it is desired to disconnect a machine 16 of an industrial system 18 (in this case, an assembly line) from a power source, or when a failure or other condition occurs at the machine 16 and an operator appropriately switches or triggers a remote lockout switch (RLS) 20 associated with that machine to an “OFF” position, the safety isolation system 10 serves to disconnect electric power and pneumatic power lines 22 and 24, respectively, from the machine so as to decouple and isolate the machine from both of those types of power. Additionally, the safety isolation system 10 may then further serve to ground the machine 16.
Once the machine 16 has been disconnected and isolated from the power source, an indication may be provided to the operator (e.g., a light 26 turns ON) indicating that it is appropriate for the operator to access the machine for purposes of making a repair or some other modification to the machine. Typically the operator will then access the machine by entering into a normally-inaccessible region, e.g., by opening a barrier 28 and entering into the machine as shown (alternatively, for example, the operator could pass through a light curtain).
Once the operator has completed the repair/modification and left the normally-inaccessible region, the operator appropriately switches or triggers the RLS 20 again, this time to an “ON” position. After this occurs, the safety isolation system 10 reestablishes the connections between the power sources and the machine 16. The safety isolation system 10 typically employs redundant circuitry such as safety relays to enhance the control system's reliability in performing its control functions in this regard.
Although control systems such as the safety isolation system 10 shown in FIG. 1 are useful, such control systems are typically designed to have only limited purpose(s) and functionality. For example, the safety isolation system 10 merely serves the purposes of disconnecting/connecting one or more loads from low voltage AC electric power (typically 600 VAC or less). In certain applications, however, it would be advantageous if such control systems could be reconfigured in a manner allowing for expanded functionality, particularly functionality involving control and monitoring of DC electrical power for DC operated equipment/machines/loads.
Despite the desirability of providing such additional functions in some circumstances, it is not possible to reconfigure conventional control systems such as the safety isolation system 10 adapted for low voltage AC electrical power to achieve such additional functions in the field. Largely this is because such conventional control systems are carefully designed for connecting and disconnecting AC electrical power and to include sufficient redundancy to enhance reliability and behave in predictable manners during failures. Reconfiguration of such conventional control systems in the field to allow for connecting and disconnecting of DC electric power loads could unpredictably alter the control systems' behavior and undermine the control systems' reliability, and consequently conventional control systems typically are designed in a manner that prevents such ad hoc reconfigurations. Some contactors (discussed below) have a DC switching rating but are limited to approximately 220 VDC and some contactors require their main contacts to be connected in series.
In addition, connecting and disconnecting of DC electrical power, especially high voltage DC (typically 220 VDC or more) and high current DC (typically hundreds of amps or more) requires special considerations due to the unique characteristics of the DC electrical power. One type of industrial automation device designed to connect/disconnect electrical power is known as a contactor. Contactors are designed for opening and closing electrical power feed lines. In an AC electrical power system, an electric current follows a waveform, typically a sine wave, and there exists a zero voltage cross over point on that waveform that helps to extinguish the arc. Because the AC voltage and current waveforms go through zero voltage and zero current, the arc problem described below that exists in DC electrical power systems will not occur.
In a DC electrical power system, there is no zero voltage cross over point. If a contactor is opened, an electric arc will form in a gas-filled space (including air) between the contacts, and without intervention will continue until the space between the electrical contacts is too large to sustain the arc. An arc can produce a very high temperature and is undesirable in most if not all industrial environments, as it can decrease the contactor's life span and can damage the contactor, including welding the contactor's main contacts.
One known solution to this arcing problem is to include an arc chute. The arc chute is used to stretch the arc a sufficient distance so that the voltage cannot support the arc, and the arc will eventually break. However in a DC system, such a contactor, including those designed specifically for DC applications, becomes undesirably large due to the size required for the arc chute and the large spacing required between the contacts within the contactor.
Another known solution to the DC arc problem is to create a hermetically sealed container to enclose the contacts. In this solution, the container is typically metal, and is typically soldered for an airtight seal. The container is then either hooked to a hard vacuum to remove air, or the container is filled with an inert gas. The absence of air decreases the distance that the arc can be maintained for the voltage in the atmosphere around the contacts. Side magnets are sometimes used in a hermetically sealed contactor to pull the arc and eventually break it. Not only does the hermetic cavity of this construction make the contactor undesirably large, it also makes the manufacture of the contactor difficult and costly.
In addition, in the last few years, the significance of safety-related circuits for both personal protection and the protection of high-value capital investments, such as industrial machinery, has become an even greater issue and an area of increased interest. The readiness for use of safety circuits exists, but there is frequently an element of uncertainty regarding the properties of contacts in the circuit, and specifically the interaction between the main contacts (for power) and the auxiliary contacts (for control). The required specifications of “mirror contacts” provide a reliable indication about the open/closed status of the main contacts of a contactor. Specifically, one requirement of a mirror contact is that a normally closed mirror contact will not change state if the main contacts weld. In the event that a main contact would weld, the normally closed mirror contact will not reclose when power is removed from the control coil of the contactor. With the special requirements as described above for DC contactors, mirror contacts have not been readily available on DC contactors. AC contactors with limited DC rating (low voltage DC and DC currents less than 420 A) are more prevalent with mirror contacts.
Given that it would be desirable for reliable, failure-resistant control systems such as the safety isolation system 10 to have the capability to connect/disconnect and isolate DC electrical power, and given that conventional systems of this type are not readily reconfigurable to provide such capabilities, it would be advantageous if an improved control system of this general type was developed that was capable of providing such capabilities. Further, it would also be advantageous if such an improved control system achieved greater levels of redundancy, reliability and failure-resistance as conventional control systems of this type through the incorporation of mirror contacts.